The invention generally relates to communication systems. More specifically, the invention relates to a method and system for determining a Log-Likelihood Ratio (LLR) corresponding to each bit of a symbol in a predetermined constellation.
LLR is generally determined to decode the raw bits. For example, LLR can be determined to decode the raw bits in a forward error correction technique so as to improve the system performance.
In known communication systems that employ a regular Quadrature Amplitude Modulation (QAM) constellation, the complexity of computing the LLRs can be reduced to a few arithmetic operations, such as addition and multiplication. These communication systems also employ rotated QAM constellation and pre-coded QAM to achieve full-diversity and/or to maximize coding gain. The method of determining LLR for a rotated QAM constellation and pre-coded QAM constellation is however, computationally intensive.
There is therefore a need for a method and system that reduces the computations involved in determining a LLR for predetermined constellations such as rotated QAM constellation and pre-coded QAM constellation.